1. Field of the Invention
The present invention relates generally to regulator circuits, and more specifically, to linear regulator circuits.
2. Background Information
There are many circuit applications that require linear regulators that are minimally degraded by noise on the power supply. Examples of these applications include high quality signal processing, signal conditioning, laptop/notebook computers, cell phones and portable headphone speaker drivers. These applications require analog circuits that exist on the same printed circuit board (PCB) as other high speed/high power circuitry and, often, very noisy switch-mode power supplies. These high speed/high power circuits tend to corrupt the power supply voltage, resulting in poor analog circuit performance. In a typical low dropout regulator design, the power supply noise appears directly at the gate or base of the pass device, since the pass device is a P-channel or PNP device whose gate or base bias is referred to the unregulated supply voltage. Such a prior art circuit using a p-channel pass transistor is shown in FIG. 1. It may be seen therein that in the first instance, the noise on the power supply Vdd appears as a gate-source voltage noise, requiring the error amplifier to respond for any control of the effect of this noise. Hence the overall line regulation is determined by the gain of the error amplifier, which may be as low as 40 dB in order to maintain system stability. Therefore there is a need to have a regulator that not only has a low dropout voltage, but also good overall line regulation.
Apparatus and methods to provide high line regulation, which can be combined with a low dropout voltage, for linear regulators. The control for the regulators is referenced to ground rather than a relatively noisy power supply terminal so that the control is substantially free of power supply noise. The pass transistor forms the second transistor of a current mirror mirroring the current from the control. Referencing the control to ground and mirroring the control current to the pass transistor makes the output of the regulator substantially independent of the power supply noise. Furthermore, the current mirror can incorporate a bias control circuit that substantially eliminates power supply induced error currents from the current mirror itself, thus further improving the line regulation. Various embodiments are disclosed.